Gas turbine engines are traditionally equipped with some type of control system, speed governor, or both. Early control systems or speed governors were mechanical or hydromechanical. FIG. 1 is a simplified schematic which shows operative principles used by a common type of mechanical speed governor.
A shaft 3, on the left side of the Figure, is connected to a linkage 6, which supports weights 9. The shaft 3 and linkage 6 rotate as indicated by arrow 12. As speed increases, the weights 9 are driven radially outward, in the directions of arrows 15 shown on the right side of the Figure. This radial motion withdraws piston 18 from a valve 21, thereby closing the valve 21 and either (1) shutting down the engine or (2) limiting the speed of the engine.
Advancements in modern electronics, and particularly in integrated circuits, have greatly (1) reduced cost, (2) increased reliability, and (3) increased the amount of functionality which can be contained in relatively small packages. For these reasons and others, the traditional mechanical control system or speed governor is being replaced by electronic control systems and overspeed protection systems.
However, despite the great benefits offered by modern electronic systems, they nevertheless suffer some disadvantages. One disadvantage is sensitivity to heat. For example, certain types of transistors can experience “thermal runaway,” wherein a high temperature promotes excessive numbers of carriers into the transistor's conduction band, thereby turning the transistor into a short circuit. The short circuited transistor attempts to conduct a very large current, and destroys itself.
Related phenomenon can occur with solid-state diodes. In addition, printed circuit boards, upon which the solid-state components are mounted, cannot withstand excessive temperatures.
Therefore, when an electronic circuit is used as part of a control system or as an overspeed protection device, in a gas turbine engine for example, the engine must be protected from erroneous control system behavior when the electronic parts of the system are exposed to overtemperature conditions.